1. Field of Invention:
This invention relates to machines to facilitate the registration and mounting of flexographic printing plates on a plate cylinder and for obtaining proofs thereof on an impression cylinder, and more particularly to a mounting-proofing machine provided with a support tray to facilitate the transfer of flexographic printing plates to desired mounting positions on the plate cylinder.
2. Status of Prior Art:
In the flexographic process, printing is conventionally effected by flexible printing plates mounted on cylinders, the paper to be printed being impressed by the inked printing plate. The cylinder on which the printing plates are mounted is generally called the plate cylinder. The quality of a flexographic printing job depends, in large measure, on the care with which pre-press preparations are carried out. Plate-mounting, color registration and proofing are effected off the press by means of commercially available mounting-proofing machines designed for this purpose.
These machines, which usually make use of an optical mounting system, make it possible to mount the plates on plate cylinders to effect exact color registration, a procedure essential to the maintenance of both quality and economy in all flexographic operations. Pre-proofing is, in many respects, the most important of all pre-press preparations, for it not only indicates the appearance of the final reproduction, but it also affords means to check the mounting of the plates for color sequence, spacing requirements, layout and gear size as well as copy and color separation.
Mounting-proofing machines are provided with a proofing cylinder (also called the impression cylinder) which cooperates with the plate cylinder, the impression cylinder making contact with the printing plates on the plate cylinder and rotating concurrently therewith to print a proof on a sheet secured to the proofing cylinder. In some commercial machines of the type presently available, the proofing or impression cylinder is supported for rotation at a fixed position, whereas the plate cylinder is movable in a vertical plane, from a mounting state in which it is retracted relative to the proofing cylinder to a proofing state in which it is in engagement therewith. One such machine is the M15 Mounter-Proofer manufactured by the Mosstype Corporation of Waldwick, N.J., the assignee of the present application. Also manufactured by this company are M17, M18, M20 and M21 mounter-proofer machines.
The M18 machine includes a line-of-sight viewer which shows the operator both the plate he is mounting on the plate cylinder and an image reflected from the proof sheet on the impression cylinder. By simply merging the two images, the operator sees exactly where to lay the plate on the plate cylinder. The entire viewer assembly automatically raises out of the way, giving unobstructed access to the plate cylinder for close inspection, inking, etc. The viewer returns automatically to the identical viewing position when ready to proceed with mounting.
The impression and plate cylinders are provided with respective gears of the same diameter. These gears are mechanically intercoupled, whereby rotation of the proofing cylinder causes the plate cylinder to rotate in both the mounting state where the cylinders are separated and in the proofing state when the cylinders are in engagement with each other. Mounter-proofer machines for rubber flexographic printing plates are disclosed in U.S. Pat. Nos. 3,361,061 (Hoexter et al.) and 4,004,509 (Moss).
When the diameter of the proofing cylinder is the same as the printing diameter of the plate cylinder (i.e., the diameter of the plate cylinder plus the thickness of the printing plates thereon), then a one-to-one relationship exists therebetween. In practice, however, the plate cylinders come in a range of diameters for different size printing jobs. This is a direct function of plate cylinder circumference which equals the pitch line circumference of the plate cylinder gear. It is necessary, therefore, in the M18 machine and in other mounting-proofing machines operating on similar principles to adjust the phase relationship between the plate and proofing cylinders to accommodate the shift in the surfaces which occur as the printing cylinder moves from its mounting to its proofing position.
Of particular prior art interest is U.S. Pat. No. 4,019,434 (Hoexter), whose entire disclosure is incorporated herein by reference. In this patent, which discloses a mounter-proofer machine, the plate cylinder is movable in a vertical plane from a proofing state in which it makes contact with the impression cylinder to a mounting state in which it is separated therefrom. An optical viewer is provided which is operative in the mounting state to show the operator both the plate he is mounting on the plate cylinder and the image reflected from the proof sheet on the impression cylinder. In order to merge these two images so that the operator sees exactly where to lay the plate on the plate cylinder, means are provided which mechanically interlink the two cylinders and also effect a relative adjustment of their angular positions whereby a given point on the plate cylinder is optically coincident with the corresponding point printed on the impression cylinder.
Mounter-proofer machines of the type disclosed in the above-identified patents are expressly designed for use with traditional flexographic printing plates formed of natural rubber and other materials that are not dimensionally stable. However, in recent years, dimensionally stable flexographic printing plates have been developed that include a synthetic polymeric ply. Such plates have made practical mechanical positioning by pin registration techniques. To this end, the printing plates have registration pin holes drilled therein which are outside of the image area and serve to properly orient the plate on the plate cylinder.
In the "CYREL Registration System" marketed by duPont and described in their bulletin E77968 bearing this title, in order to properly mount a photopolymer plate onto a plate cylinder having a sticky surface to which the plate is adherable, a registration bar is provided having end plates that clamp onto the bearing surfaces of the plate cylinder.
The registration bar is provided with registration pins, the bar when clamped being above and parallel to the plate cylinder. The printing plate to be mounted on the plate cylinder is draped over the bar, with the registration pins on the bar going into the pin holes on the printing plate to properly orient the printing plate whose front portion then makes contact with and adheres to the sticky surface on the plate cylinder. The printing plate is then taken off the bar pins, after which the bar is detached from the plate cylinder and the remaining portion of the printing plate is then smoothed onto the cylinder so that it is now fully mounted thereon.
In the Cyrel Registration System there is a need to manually clamp the registration bar to the plate cylinder each time a printing plate is to be mounted thereon. One must then subsequently remove the bar and then move the plate cylinder to a proofing position for final proofing.
Accordingly, the need exists for a proofing machine that incorporates the features of pin registration to facilitate pin mounting methods in a production environment, and for obtaining proofs thereof to check the mounting of the plates for color sequence, spacing requirements, and for other factors which must be taken into account in high quality flexographic printing.
While the invention will be described in connection with photopolymer flexographic printing plates having registration holes drilled therein, it is to be understood that it is applicable to other types of dimensionally-stable flexographic printing plates provided with registration holes.